Conventionally, there has been a nonvolatile memory capable of storing two bits by one field-effect transistor (e.g., see JP-2001-512290 A, published Aug. 21, 2001). The structure of this nonvolatile memory and the principle of its write (program) operation will be described hereinbelow.
As shown in FIG. 25, this memory is composed of a gate electrode 909 formed on a P type well region 901 through a gate insulating film, and a first N type diffusion layer region 902 and a second N type diffusion layer region 903 formed at the surface of the P type well region 901. The gate insulating film is composed of a so-called ONO (Oxide Nitride Oxide) film in which a silicon nitride film 906 is interposed between silicon oxide films 904 and 905. In the silicon nitride film 906, there are formed memory holding portions 907, 908 in the vicinity of the edge portions of the first and second N type diffusion layer regions 902, 903. An electric charge amount in each of these memory holding portions 907, 908 is read as a drain current of the transistor so that two-bit information is stored in one transistor.
Next description will be given of a write operation method in this nonvolatile memory. The term “write (or program)” is used herein to refer to the action of injecting electrons into the memory holding portion 907, 908. In JP-2001-512290 A, there has been disclosed a method for injecting electrons into a right memory holding portion 908 by applying 5.5V to the second diffusion layer region 903 and 10V to the gate electrode 909. This makes it possible to write to a specific one of the two memory holding portions. There has been also disclosed a method for erasing and reading from a specific side. By combining these methods, two-bit operation is enabled.
In the above-stated nonvolatile memory, in order to provide the gate insulating film with the function of operating the transistor as well as the function as a memory film for storing electric charges, the gate insulating film is formed into three-layer structure with use of the ONO film. This makes it difficult to manufacture thinner gate insulating films. Also, in the above nonvolatile memory, as the channel length is shortened, the two memory holding portions 907, 908 in one transistor interfere with each other, which makes two-bit operation difficult. This obstructs further miniaturization of the devices.